You have discovered a novel, negatively charged protein that binds strongly to t

Question

You have discovered a novel, negatively charged protein that binds strongly to the positive charges on the amino-terminal tails of histone proteins. You have conducted an experiment where you purify nucleosomes from cells and then add high levels of your novel protein. You observed using electron microscopy that this treatment tends to cause the nucleosomes to convert from a 30nm fiber configuration into a 10nm beads-on-a-string configuration (as compared to an untreated control sample). (1) Please propose a molecular mechanism for why your novel protein would have this effect. (2) Do you think that the presence of your novel protein would tend to increase or decrease gene expression?

Explanation / Answer

1. Histone protein H1 that is linker has a its important role in bringing close the other histone and form a beads on string structure of 30nm.

Histone H1 has tripartite structure consisting of a globular domain flanked by two lysine-rich tails, a shorter amino-terminal domain (N-tail) and a longer carboxyl-terminal one (C-tail).Despite positioning along the nucleosome core particle, it is not the globular domain but rather the highly positively charged C-tail that imparts to linker histones their unique ability to bind to linker DNA through nonspecific electrostatic interactions.

So, If we will high level of our novel protein to the nucleosome then it will bind to the long Cterminal tail of H1 protein.

This will lead to the unavailability of the C tail for Chromatin Binding. From literature it is also reported that in vivo, the absence of C-tails leads to greatly reduced chromatin binding. Binding of linker histones to the linker DNA facilitates the shift of the chromatin structure toward more condensed, higher order forms.

In this way this will effect the length of the nucleosome and we will get a 10nm nucleosome instead of 30 nm.

2.Due to excess of negative charge on the histone protein , when it will interact to DNA ,there will be repulsion between them and hence there will be a loose interaction between both.

Secondly Nucleosomes are obstacles to the elongating RNA polymerase, which may need to transfer the histone octamers it encounters to acceptor DNA in the wake of elongation Due to loosen structure of the nucleosome there will be less hinderance to the Redulators during transcription to proced.Hence the gene expression will be enhanced.

It is already said that Acetylation at lysine residue neutralises the positive charge and hence there will be loosen interaction between histone and DNA. Our negatively charged protein is also in excess, so some of it will also bind to Lysine and hence it will further help in neutralising positive charge, and there will be a great increase in gene expression.

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